a) Field of the Invention
The invention relates to an arrangement for balancing the beam of one or more high-power diode lasers for the purpose of achieving high power densities in simultaneous conjunction with high beam quality in accordance with the preamble of patent claim 1.
b) Description of the Related Art
The emission characteristics of high-power diode lasers (diode lasers with a plurality of emitters or emitter groups arranged in a row) (termed HP-DL below) have been sufficiently frequently described. The special feature which arises with HP-DLs is the fact of the strongly differing divergence of the emitted beam and the spatial extent of the emitter arrangement in a plane parallel and perpendicular to the pn junction. Thus, in the plane of the pn junction (fast, axis below) the divergence is approximately 45 degrees FHWM (full width at half minimum), and in the plane perpendicular to the pn junction (slow axis below) it is approximately 10 degrees FHWM.
A radiation field of 1 xcexcm height and 200 mm width is yielded, for example, given a row of 25 emitters which are at a distance of approximately 200 xcexcm from one another, in the case of typical dimensions for an emitter of approximately 1 xcexcm heightxc3x97200 xcexcm width. The emitted beam is inhomogeneous over the width of the radiation field, that is to say it comprises a plurality of beams determined by number, distance and width of the emitters. For the emitter arrangement quoted by way of example, the result is 25 beams with a beam exit aperture of 1 xcexcmxc3x97200 xcexcm. These beams can be combined in a complex or in groups, or else be shaped individually for the purpose of balancing the beam.
In the direction of beam propagation and starting from the HP-DLs, all known optical arrangements for balancing the beam of HP-DLs have in each case as first optical element a cylindrical lens, arranged upstream of a row of emitters, for collimating the beam in the direction of the fast axis. Different combinations of optical elements and assemblies are arranged below with the aim of realizing homogenization of the strongly differing beam qualities of the two planes.
Such arrangements are known from WO 96/13884, DE 196 45 150 and DE 195 00 513. The two first named solutions have the disadvantage that the number of the HP-DLs whose beam can be projected suitably into the arrangement is greatly limited, as a result of which the achievable total power of the HP-DL is limited to approximately 100 W.
The arrangement presented in DE 195 00 513 can be applied for HP-DLs having a large number of emitters, chiefly for a plurality of HP-DLs arranged one above another. This arrangement permits scaling to very high powers through the beam shaping of very many emitters. In the arrangement described in DE 195 00 513, the first step, as is known, is to collimate the beam of each individual HP-DL to a residual divergence of a few mrad in the direction of the fast axis by means of a cylindrical lens.
Because of the juxtaposition of a plurality of individual emitters or emitter groups, the beam in the direction of the slow axis is, as already mentioned, not a homogeneous line radiation source.
Since line radiation sources can be collimated or focused the better with the aid of an optical arrangement the smaller the extent of the line source, it is advantageous for the beams of the individual emitters or emitter groups to be collimated individually in the direction of the slow axis.
For this purpose, in DE 195 00 513 a prism system is arranged downstream of a cylindrical lens in each case in order to deflect the beam, collimated in the direction of the fast axis, of adjacent emitters or emitter groups in the direction of the fast axis. The partial beams thus deflected remain offset relative to one another in the direction of the slow axis. As a result, the cylindrical lens elements, assigned to the individual beams, of a segmented collimation lens system for collimating the beam in the direction of the slow axis can be arranged in an overlapping fashion with reference to the slow axis. Consequently, the refraction losses can be minimized by using larger collimation lenses even in the case of small distances between the emitters or emitter groups of an HP-DL. Good beam formation is rendered possible in the direction of the slow axis, but the segmented cylindrical lens system entails a substantial outlay on implementation.
The beam quality of the individual beams separated by the deflection described decisively determines the beam quality of the overall optical arrangement.
The optical system according to DE 195 00 513 does not, however, effect an optimum rearrangement of the beams from the point of view of beam quality:
The original offset of the separated individual beams along the slow axis is maintained, and there is no balancing of the beam quality by rearrangement in the two axial directions. Consequently, the beam quality, that is to say the achievable minimum focal range or the numerical aperture of the entire beam upon exiting from the arrangement is bounded below by the system.
However, there is a very high outlay on forming the beam in the direction of the slow axis owing to the insertion of a segmented collimation lens system.
It is the primary object of the present invention to exhibit a new simpler optical arrangement which in addition to the efficient beam formation for the beam of one or more HP-DLs also carries out a beam rearrangement in such a way that balancing of the beam quality is achieved along the slow and fast axes before focusing, as a result of which the beam quality is substantially improved at the focus.
This object is achieved with the aid of an optical arrangement in accordance with the invention for balancing the beam of one or more high-power diode lasers, arranged one above another, for the purpose of focusing at a point. The arrangement includes a high power diode laser having a plurality of emitters or emitter groups arranged in a row and a first cylindrical lens arranged downstream of each row for collimating the beam of the emitters or emitter groups in a direction of the pn junction of the emitters (fast axis). Also included is a direction element with a number of different segments equal to the number of emitters or emitter groups, which deflect the beams in the direction of the fast axis by diffraction, refraction or reflection. Means are provided for collimating the beams in the direction perpendicular to the pn junction of the emitters (slow axis). A further lens focuses the entire beam at a point, the focus being situated on the optical axis, which is defined as the perpendicular to the direction of the slow axis and the fast axis. The means for collimating the beams in the direction perpendicular to the pn junction (slow axis) is a single second cylindrical lens. Segments of the direction element deflect the beams in the direction of the slow axis relative to one another in each case such that their principal rays overlap with the optical axis at a distance A from the direction element in the direction perpendicular to the pn junction and are arranged one above another at equal distances in the direction of the pn junction. A redirection element is arranged downstream at the distance A from each direction element which deflects the beams such that they run parallel to one another.
It is essential to the invention that the beams of the individual emitters or emitter groups of each individual HP-DL are deflected by optically diffracting, reflecting or refracting means such that they are superimposed in the direction of the slow axis so that they can be collimated in the direction of the slow axis with the aid of a single cylindrical lens, and substantial balancing of the overall beam is achieved.
The arrangement according to the invention is described below in more detail with the aid of the drawings.